Commentary: On the merit of an early contributor of the "Preparation for Oxidative Stress" (POS) theory

Fish models to explore epigenetic determinants of hypoxia-tolerance

The occurrence of environmental hypoxia in freshwater and marine aquatic systems has increased over the last century and is predicted to further increase with climate change. As members of the largest extant vertebrate group, freshwater fishes, and to a much lesser extent marine fishes, are vulnerable to increased occurrence of hypoxia. This is important as fishes render important ecosystem services and have important cultural and economic roles. Evolutionarily successful, fishes have adapted to diverse aquatic freshwater and marine habitats with different oxygen conditions. While some fishes exhibit genetic adaptions to tolerate hypoxia and even anoxia, others are limited to oxygen-rich habitats. Recent advances in molecular epigenetics have shown that some epigenetic machinery, especially histone- and DNA demethylases, is directly dependent on oxygen and modulates important transcription-regulating epigenetic marks in the process. At the post-transcriptional level, hypoxia has been shown to affect non-coding microRNA abundance. Together, this evidence adds a new molecular epigenetic basis to study hypoxia tolerance in fishes. Here, we review the documented and predicted changes in environmental hypoxia in aquatic systems and discuss the diversity and comparative physiology of hypoxia tolerance in fishes, including molecular and physiological adaptations. We then discuss how recent mechanistic advances in environmental epigenetics can inform future work probing the role of oxygen-dependent epigenetic marks in shaping organismal hypoxia-tolerance in fishes with a focus on within- and between-species variation, acclimation, inter- and multigenerational plasticity, and multiple climate-change stressors. We conclude by describing the translational potential of this approach for conservation physiology, ecotoxicology, and aquaculture.

REVIEW: Evidence supporting the 'preparation for oxidative stress' (POS) strategy in animals in their natural environment

Hypometabolism is a common strategy employed by resilient species to withstand environmental stressors that would be life-threatening for other organisms. Under conditions such as hypoxia/anoxia, temperature and salinity stress, or seasonal changes (e.g. hibernation, estivation), stress-tolerant species down-regulate pathways to decrease energy expenditures until the return of less challenging conditions. However, it is with the return of these more favorable conditions and the reactivation of basal metabolic rates that a strong increase of reactive oxygen and nitrogen species (RONS) occurs, leading to oxidative stress. Over the last few decades, cases of species capable of enhancing antioxidant defenses during hypometabolic states have been reported across taxa and in response to a variety of stressors. Interpreted as an adaptive mechanism to counteract RONS formation during tissue hypometabolism and reactivation, this strategy was coined "Preparation for Oxidative Stress" (POS). Laboratory experiments have confirmed that over 100 species, spanning 9 animal phyla, apply this strategy to endure harsh environments. However, the challenge remains to confirm its occurrence in the natural environment and its wide applicability as a key survival element, through controlled experimentation in field and in natural conditions. Under such conditions, numerous confounding factors may complicate data interpretation, but this remains the only approach to provide an integrative look at the evolutionary aspects of ecophysiological adaptations. In this review, we provide an overview of representative cases where the POS strategy has been demonstrated among diverse species in natural environmental conditions, discussing the strengths and weaknesses of these results and conclusions.

Effects of environmental hypoxia on the goldfish skeletal muscle: Focus on oxidative status and mitochondrial dynamics

The skeletal muscle is a highly plastic tissue. Its ability to respond to external stimuli and challenges allows it to face the functional needs of the organism. In the goldfish Carassius auratus, a model of hypoxia resistance, exposure to reduced oxygen is accompanied by an improvement of the swimming performance, relying on a sustained contractile behavior of the skeletal muscle. At the moment, limited information is available on the mechanisms underlying these responses. We here evaluated the effects of short- (4 days) and long- (20 days) term exposure to moderate water hypoxia on the goldfish white skeletal muscle, focusing on oxidative status and mitochondrial dynamics. No differences in lipid peroxidation, measured as 2-thiobarbituric acid-reacting substances (TBARS), and oxidatively modified proteins (OMP) were detected in animals exposed to hypoxia with respect to their normoxic counterparts. Exposure to short-term hypoxia was characterized by an enhanced SOD activity and expression, paralleled by increased levels of Nrf2, a regulator of the antioxidant cell response, and HSP70, a chaperone also acting as a redox sensor. The expression of markers of mitochondrial biogenesis (TFAM) and abundance (VDAC) and of the mtDNA/nDNA ratio was similar under normoxia and under both short- and long-term hypoxia, thus excluding a rearrangement of the mitochondrial apparatus. Only an increase of PGC1α (a transcription factor involved in mitochondrial dynamics) was detected after 20 days of hypoxia. Our results revealed novel aspects of the molecular mechanisms that in the goldfish skeletal muscle may sustain the response to hypoxia, thus contributing to adequate tissue function to organism requirements.

Glutathione Depletion Disrupts Redox Homeostasis in an Anoxia-Tolerant Invertebrate

The upregulation of endogenous antioxidants is a widespread phenomenon in animals that tolerate hypoxia/anoxia for extended periods. The identity of the mobilized antioxidant is often context-dependent and differs among species, tissues, and stresses. Thus, the contribution of individual antioxidants to the adaptation to oxygen deprivation remains elusive. This study investigated the role of glutathione (GSH) in the control of redox homeostasis under the stress of anoxia and reoxygenation in Helix aspersa, an animal model of anoxia tolerance. To do so, the total GSH (tGSH) pool was depleted with l-buthionine-(S, R)-sulfoximine (BSO) before exposing snails to anoxia for 6 h. Then, the concentration of GSH, glutathione disulfide (GSSG), and oxidative stress markers (TBARS and protein carbonyl) and the activity of antioxidant enzymes (catalase, glutathione peroxidase, glutathione transferase, glutathione reductase, and glucose 6-phosphate dehydrogenase) were measured in foot muscle and hepatopancreas. BSO alone induced tGSH depletion by 59-75%, but no other changes happened in other variables, except for foot GSSG. Anoxia elicited a 110-114% increase in glutathione peroxidase in the foot; no other changes occurred during anoxia. However, GSH depletion before anoxia increased the GSSG/tGSH ratio by 84-90% in both tissues, which returned to baseline levels during reoxygenation. Our findings indicate that glutathione is required to withstand the oxidative challenge induced by hypoxia and reoxygenation in land snails.

Short-Term Estivation and Hibernation Induce Changes in the Blood and Circulating Hemocytes of the Apple Snail Pomacea canaliculata

States of natural dormancy include estivation and hibernation. Ampullariids are exemplary because they undergo estivation when deprived of water or hibernation when exposed to very low temperatures. Regardless of the condition, ampullariids show increased endogenous antioxidant defenses, anticipating the expected respiratory burst during reoxygenation after reactivation, known as "Preparation for Oxidative Stress (POS)". In this work, we tested the POS hypothesis for changes in the blood and hemocytes of the bimodal breather Pomacea canaliculata (Ampullariidae) induced at experimental estivation and hibernation. We described respiratory (hemocyanin, proteins, lactate), antioxidant (GSH, uric acid, SOD, CAT, GST), and immunological (hemocyte levels, ROS production) parameters. We showed that, although the protein level remains unchanged in all experimental groups, hemocyanin increases in response to estivation. Furthermore, lactate remains unchanged in challenged snails, suggesting an aerobic metabolism during short-term challenges. Blood uric acid increases during estivation and arousal from estivation or hibernation, supporting the previously proposed antioxidant role. Regarding hemocytes, we showed that the total population increases with all challenges, and granulocytes increase during hibernation. We further showed that hibernation affects ROS production by hemocytes, possibly through mitochondrial inhibition. This study contributed to the knowledge of the adaptive strategies of ampullariids to tolerate adverse environmental conditions.

The role of solar radiation and tidal emersion on oxidative stress and glutathione synthesis in mussels exposed to air

Preparation for oxidative stress (POS) is a widespread adaptive response to harsh environmental conditions, whose hallmark is the upregulation of antioxidants. In contrast to controlled laboratory settings, animals are exposed to multiple abiotic stressors under natural field conditions. Still, the interplay between different environmental factors in modulating redox metabolism in natural settings remains largely unexplored. Here, we aim to shed light on this topic by assessing changes in redox metabolism in the mussel Brachidontes solisianus naturally exposed to a tidal cycle. We compared the redox biochemical response of mussels under six different natural conditions in the field along two consecutive days. These conditions differ in terms of chronology, immersion/emersion, and solar radiation, but not in terms of temperature. Animals were collected after being exposed to air early morning (7:30), immersed during late morning and afternoon (8:45-15:30), and then exposed to air again late afternoon towards evening (17:45-21:25), in two days. Whole body homogenates were used to measure the activity of antioxidant (catalase, glutathione transferase and glutathione reductase) and metabolic (glucose 6-phosphate dehydrogenase, malate dehydrogenase, isocitrate dehydrogenase and pyruvate kinase) enzymes, reduced (GSH) and disulfide (GSSG) glutathione levels, and oxidative stress markers (protein carbonyl and thiobarbituric acid reactive substances). Air and water temperature remained stable between 22.5 °C and 26 °C during both days. Global solar radiation (GSR) greatly differed between days, with a cumulative GSR of 15,381 kJ/m² for day 1 and 5,489 kJ/m² for day 2, whose peaks were 2,240 kJ/m²/h at 14:00 on day 1 and 952 kJ/m²/h at 12:00 on day 2. Compared with animals underwater, emersion during early morning did not elicit any alteration in redox biomarkers in both days. Air exposure for 4 h in the late afternoon towards evening caused oxidative damage to proteins and lipids and elicited GSH synthesis in animals that had been previously exposed to high GSR during the day. In the following day, when GSR was much lower, exposure to air under the same conditions (duration, time, and temperature) had no effect on any redox biomarker. These findings suggest that air exposure under low-intensity solar radiation is not sufficient to trigger POS in B. solisianus in its natural habitat. Thus, natural UV radiation is possibly a key environmental factor that combined to air exposure induces the POS-response to the stressful event of tidal variation in this coastal species.